Is it advisable to run yum -y daily via a cronjob on a critical production server?

To get and install daily all the last updates, new features, bug fixes, and security fixes.

Is there any "industry standard" regarding this?

Certainly not.

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Unattended upgrades on a working production server?

Turn in your Linux toaster.

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What they said ...
Get yourself a test machine & do it on there first....

Always test updates before you apply them to production. Linux Admin 101.

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and "-y" is very very very dangerous
use that with EXTREME CAUTION

it is "yes to EVERYTHING and do not inform me as to changes -- just DO IT "
and it will do what ever "it" is

You can help minimize your risk of an update causing issues by only installing security updates on productions servers. This mean NOT installing bug/feature updates.
Only install bug fixes if needed.

To make yum only install security patches do the following:
1. Install the yum security plugin by running 'yum install yum-plugin-security'
2. Add the security option to yum update command like this 'yum --security update'

So, if you do NEED to automate your updates (THIS IS NOT ADVISABLE), installing security only patches will help minimize your risks.
BTW, I only install security patches on all my production servers as a rule, even tho I run them manually.

Hope this helps.

... but, even then, you might be "relying too much" upon what some distro-vendor considers to be "a 'security' update."

The single most-important attribute of any production server is: "absolute continuity of service." It is not okay(!) to update the software that performs that service, "any ol' time a distro-provider wants to." You must plan(!) for these things.

While I agree with you, that's not the whole story. I did not go into how I deal with the issue of continuity. In my environment, I take snapshots of anything I'm going to run update's on. This is beyond our normal backups, test environments, etc. that we have in place. Having said that, I must, at some point, either trust the updates, exhaustively scan thru every line of source code (windows source code??? ..... anyone???) or not run the updates. I have found that even running updates in a test env. doesnt always catch all the issues one can run into. Fortunately, Redhat is very good concerning its security updates (I dont think I've ever had a RH security update brake something) and snapshots allows me to run updates then hit the rewind button if something brakes. ...... usually :}

But, again, you make a very good point... thanks!

I think the clue to answer is this "a critical production server".

You are really in a tough boat on this. You NEED to keep it current for threats but you also don't need to bork it with a faulty update of some junk program.

Best practice is to look at updates and decide or get management to issue a rule.

One clever "shop" that I know of puts all "system-related" files onto a single volume (image). Every part of any of their "standard server-configurations" which is not "malleable user data" is stored on that volume.

There are three virtual-disks associated with each (virtual ...) production server: "previous," "current," and "next." All of them are read-only to the server in question.

Regularly, they apply system updates using an offline master machine, then run a series of production-readiness tests which gruelingly examine everything that is of operational importance to them.

If everything is well, the changes are replicated via rsync onto the "next" images.

A server can then be shut-down, the three disk-images are "rotated," and the server is restarted. The previous "last" image becomes the "next" one.

And, in the case of "extreme unction" , they have one, if not two, "fall-back positions" to drop to.

I like this idea a lot. Would like to know more about this procedure.

I would like to know more too.